The present invention relates to an automatic focussing device and more particularly, although not exclusively, to an automatic focussing device suitable for use with an image analysis system, such as an image analysis system used to analyse the nature of cells present on a slide. Such a device may be used, for example, in a cervical cytolosy pre-screening cytolosy programme. Whilst such an arrangement will incorporate a microscope to be focussed, the invention may find applications in other devices which require optical focussing such as projectors, cameras and the like.
It has been proposed previously to provide an automatic focussing device for use in such an image analysis sytem. The device consisted of a beam splitter attachment for use with an appropriate microscope, the beamsplitter serving to split the image beam into three separate beams. Each part of the split beam is directed towards a respective image sensor. One sensor is located on the main focal plane of the relevant beam, and the other two sensors are located respectively just short of and just beyond the main focal plane of the respective beams.
A focus error signal is derived from the sensors which consists of a suitable non-linear function of illumination which favours dark/light transitions at the edges of the objects and within objects. This is integrated for each sensor over the same field of view, since at any instant the same image portion is directed towards the three sensors. This integral will be a maximum for a sensor on which an image is sharply focussed, and the value of the integral provided by two sensors which are equally out of focus will be the same. Consequently the focussing of the arrangement can be adjusted so that the value of the integral derived from the main sensor is at a maximum, and the value of the integrals obtained from the remaining two sensors is eual.